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www.fairchildsemi.com AN5235 65V Fast Recovery SuperFET II MOSFET for High System Efficiency and Reliability in Resonant Topologies Abstract Telecom and sever power systems are required to deliver more power within smaller volume due to limited space and increased power consumption. Under rapidly changing circumstances that encourage energy saving, most industry experts agree that new power technologies can play a critical role in the power conversion applications. The superjunction MOSFET enables a dramatically reduced onresistance compared to conventional MOSFETs thanks to its charge balance structure. Since conduction losses are directly proportional to onresistance of MOSFET, superjunction MOSFETs can greatly reduce conduction loss in system. Therefore, superjunction MOSFETs have been used in resonant converters to increase system efficiency, but generally, its body diode performance is not attractive for these topologies. Newly developed 65V fast recovery superjunction MOSFETs, called SuperFET II FRFET MOSFET have fast body diode, higher threshold voltage (Vth=4 V), ultra low onresistance, low stored energy in output capacitance and extremely fast switching speed. It can provide improved reliability and efficiency in server and telecom power applications. Introduction Distributed power systems are under pressure continuously to achieve high efficiency and reduce energy consumption in server and telecom power supplies. The increasing efficiency and power density is enabled by the continuous development of novel resonant topologies and outstanding power devices which allow a system reliability and a higher switching frequency at relatively low switching losses, which leads to a reduced converter dimensions. Several soft switching topologies for server and telecom power supplies have been introduced to reduce switching losses and device stress while achieving high power density and improved reliability. However, power MOSFET failures have been issued in the phaseshifted ZVS fullbridge topology and LLC resonant topology. As shown in Figure 1, reverse recovery of planar MOSFET is relatively softer than that of superjunction MOSFET. When all situations are same, snappy body diodes always cause higher voltage spikes and dv/dt which cause device failure. Soft body diode of Planar MOSFET is suitable for resonant topologies. However, low R DS(ON) and stored energy in output capacitance, E OSS of the MOSFET is critical factor for resonant converters to maximize system efficiency. Furthermore, low Q RR and robust body diode characteristics are related to the reliability issues. Therefore, lower R DS(ON) and E OSS with robust body diode of fast recovery superjunction MOSFETs can effectively minimize resonant energy required to achieve soft switching without increasing the circulating energy and improve the system reliability. Figure 1. Reverse recovery behavior comparison between Planar MOSFET and Superjunction MOSFET (test condition: Vdd=4 V, di/dt=1 A/µs, Isd=2 A) Rev. 1. 7/3/15
AN5235 65V Fast Recovery SuperFET II MOSFET Technology SuperFET II MOSFET combines faster switching and Qrr of body diode performance with R DS(ON) reduced by 4% compared to previous generation superjunction MOSFETs called SuperFET I MOSFETs as shown in Figure 2 [1]. As shown in Table 1, the gate charge, Qg of 65 V/19 mω SuperFET II FRFET MOSFET is dramatically reduced by 27% compared to previous generation 6 V/19 mω SuperFET I FRFET MOSFET. Normalized Specific R DS(ON) 1.2 1.8.6.4.2 6V Conventional MOSFET 6V SuperFET I MOSFET 6V SuperFET II MOSFET Figure 2. Normalized onresistance per specific area comparisons Table 1. Critical Specification Comparison DUTs BVDSS R DS(ON) Max. Qg Max. Trr Typ. Qrr Typ. E OSS @ 4VDS SuperFET II FRFET MOSFET, FCP19N65F 65 V 19 mω 78 nc 15 ns 515 nc 6.55 µj SuperFET I FRFET MOSFET, FCA2N6F 6 V 19 mω 98 nc 16 ns 1,1 nc 8.8 μj Low E OSS in Resonant Topologies Zero voltage switching (ZVS) topologies can achieve lossless turnon while drainsource voltage is zero by flowing current through the body diode during dead time as shown in Figure 3. Dead time V gs_ls Dead time V gs_hs inductance of the transformer, to discharge the output capacitance of the switches through resonant action. v IN Vds1 Q1 Ipri N : 1 Cr L r 1 Q2 ILm L m NV o N 1 SR1 V o R LOAD Figure 3. I ds_ls V ds_ls ZVS Operation Modes of Power MOSFET in LLC Resonant Converter MOSFET output capacitance is another crucial parasitic parameter to understand for zero voltage switching (ZVS) topologies. It determines how much inductance is required to provide ZVS conditions because MOSFET output capacitance can be used as a resonant component in soft switching topologies. In the soft switching topologies, zero voltage turnon is achieved by using the energy stored in inductor, the leakage and series inductance or magnetizing t SR2 Figure 4. LLC Resonant Converter The inductance should be precisely designed to prevent hard switching that causes additional power losses. LLC resonant halfbridge converter topology is shown in Figure 4. The following equation (1) and (2) shows ZVS requirements for LLC resonant converter in Figure 5. 1 2 1 2 Leq I p 2 COSS( er ) V (1) IN 2 2 1 Leq I 2 p 2 E (2) OSS 2 Where, C oss(er) is energy related output capacitance of Q 1 or Q 2 at V IN, L eq is equivalent inductance. Rev. 1. 7/3/15 2
Eoss [uj] AN5235 C oss1 v in C oss2 C r L r nv o n I 2 R L Lm Figure 5. Resonant components for ZVS mode in LLC Resonant Converter The output capacitance plays an important role in soft switching topologies. Magnetizing current must be large enough to discharge the C OSS of MOSFET during dead time to ensure the ZVS turnon as shown in equation (3). Low output capacitance of MOSET can reduce the need of magnetizing current, less circulating energy, less turnoff loss and less dead time L t T d O m (3) 16 Coss Figure 6 shows operation waveforms in LLC resonant converter according to L m with f s < f r. The peak magnetizing current (I P ) shows in equation (4) [2] I p NVo TO (4) L 4 m Where N is the transformer turnsratio, V O is the output voltage, Lm is the magnetizing inductance, t d is dead time, and T O is the switching period. Figure 6. (b) Waveforms with Small L m (f s < fr) Waveforms in LLC Resonant Converter according to Lm As shown in Figure 6, the turnoff current of the MOSFETs is determined by magnetizing inductance. With Small Lm, high peak magnetizing current will increase turnoff current of the primary side MOSFETs and circulating current. The increased current results in higher turnoff switching loss and conduction loss respectively. To achieve minimum conduction loss and turnoff loss, a large Lm is preferred. As shown in Figure 7, a SuperFET II FRFET MOSFET has approximately 23.3% less stored energy in output capacitance than SuperFET I FRFET MOSFET at 4 V across the MOSFET. Figure 8 shows the switching losses comparison. A SuperFET II FRFET MOSFET has much better switching performance, that is 22~42% less switching losses according to load current, compared to previous generation SuperFET I FRFET MOSFET in clamped inductive switching test under the following test condition : Vdd=4 V, Rg=4.7 ohm and Id=2~2 A 15 65V SuperFET II FRFET MOSFET, FCP19N65F 6V SuperFET I FRFET MOSFET, FCA2N6F T o /4 1 I pri T o I Lm I Lm I p1 5 1 2 3 4 5 6 V DS, DrainSource Voltage [V] * Notes : 1. V GS = V 2. f = 1 MHz V DS1 t d (a) Waveforms with Large L m (f s < fr) Figure 7. Comparisons of stored energy in output capacitance, EOSS Rev. 1. 7/3/15 3
Switching Losses [uj] AN5235 4 35 3 65V SuperFET II FRFET MOSFET, FCP19N65F 6V SuperFET I FRFET MOSFET, FCA2N6F 25 2 15 1 5 2 4 6 8 1 12 14 16 18 2 Id [A] Id_Q1:2A/div Vds_Q1:1V/div Id_Q2:2A/div Vds_Q2:1V/div 1us/div Figure 8. Comparisons of switching losses (Eon Eoff) under Vdd=4 V, Rg=4.7 ohm and Id=2~2 A Figure 9. Waveforms of Power MOSFETs at Startup Robust Body Diode in Resonant Topologies One of the MOSFET failure modes in LLC resonant converter is losing ZVS in abnormal conditions. Figure 9 shows waveforms of the power MOSFETs in LLC resonant converter at startup. The LLC resonant converter requires a device with body diode ruggedness characteristic because there is high current stress in over load, output short circuit condition and inrush current during startup. In startup condition, Peak inrush currents can be several orders of magnitude greater than the normal current in steady state condition. These inrush currents flow through the body diode of lowside MOSFET during start up. It makes shootthrough problem when highside MOSFET is turnedon due to reverse recovery current, which flows through body diode of lowside MOSFET. As a result, the potential failure of power MOSFET may happen during body diode reverse recovery at startup state. And another field failure can be occurred at overload or shortcircuit condition in the LLC resonant converter. Even though voltage and current of power MOSFETs are within safe operating area, some unexpected failures associated with shoot through current, reverse recovery dv/dt, and breakdown dv/dt happen in various conditions, such as over load and output short circuit. The worst case is a shortcircuit condition. During short circuit, the MOSFET conducts extremely high (theoretically unlimited) current. When short circuit occurs, operation mode during short circuit is almost same as overload condition, but shortcircuit condition is worse because reverserecovery current, which flows through the body diode of the switch, is much higher [3][4]. Figure 1. Waveforms of Power MOSFET at Output Short Condition Figure 1 shows the waveforms of the power MOSFETs in LLC resonant converter at short circuit condition. The current level during shortcircuit condition is much higher and can lead to increased junction temperature of MOSFET, which makes it easier to fail. Body diode reverse recovery is switching process of the body diode from on state to reverse blocking state. First, the body diode was forwardconducted for a while. During this period, charges are stored in the PN junction of the diode. When reverse voltage is applied across the diode, stored charge should be removed to go back to blocking state. The removal of the stored charge occurs via two phenomena: the flow of a large reverse current and recombination. A large reverserecovery current occurs in the diode during the process. This reverserecovery current flows through the body diode of MOSFET because the channel is already closed. Some of reverse recovery current flows right underneath N source. Basically, base and emitter of parasitic BJT are shorted together by source metal. Therefore, the parasitic BJT should not be activated. In practice, however, the small resistance works as base resistance. When large current flows through R b, a voltage across R b that acts as baseemitter forward bias becomes high enough to trigger the parasitic BJT. Once the parasitic BJT turns on, a hot spot is formed and more current crowds into it. More current flows through it due to negative temperature coefficient of the Rev. 1. 7/3/15 4
Drain to Source Voltage, Vds[V] ISD[A] AN5235 BJT. Finally, the device fails. Body diode conduction should be minimized to lower peak reverserecovery current. As the di/dt becomes bigger, peak reverserecovery current goes up as well. In the LLC resonant converter, the di/dt of one power MOSFET body diode is related to turnon speed of the other complementary power switch. So, slowing down the turnon also lowers the di/dt. Fast recovery MOSFET can prevent this failure thanks to its robust body diode performance Figure 12 shows competitor s fast recovery MOSFET failing waveforms during body diode reverse recovery. With competitor, failure occurs after the current level reaches I rrm, peak reverse recovery current at 8 A/µs. As shown in Figure 13, SuperFET II FRFET MOSFET did not fail at even higher di/dt (1,2 A/µs) conditions. SuperFET II FRFET MOSFET provides soft and rugged body diode during hard commutation of body diode. 16 12 65V SuperFET II FRFET MOSFET, FCP19N65F 6V SuperFET I FRFET MOSFET, FCA2N6F 8 4 4 8 12 16 2.n 1.n. 1.n 2.n 3.n Time [ns] (a) Measured I SD during Reverse Recovery Behavior of Body Diode Figure 12. Competitor s Fast recovery MOSFET Failing Waveforms During Body Diode Reverse Recovery 4 65V SuperFET II FRFET MOSFET, FCP19N65F 6V SuperFET I FRFET MOSFET, FCA2N6F 3 2 1 2.n 1.n. 1.n 2.n 3.n Time [ns] (b) Measured V DS during Reverse Recovery Behavior of Body Diode Figure 11. Comparisons of reverse recovery behavior under ISD=1 A, di/dt=1 A/μs, VDS=4 V, Tj=25ºC Figure 11 shows the reverse recovery behavior comparison at I SD =1 A, di/dt=1 A/μs, V DS =4 V and T j =25ºC. It can be clearly seen that the reverse recovery charge, Q rr of SuperFET II FRFET MOSFET, FCP19N65F reduced by 47% compared to SuperFET I FRFET MOSFET, FCA2N6F. Furthermore, peak voltage spikes of a SuperFET II FRFET MOSFET during reverse recovery behavior is lower than previous generation due to its soft reverse recovery characteristics and small Q rr. Figure 13. SuperFET II FRFET MOSFET Withstanding Waveforms During Body Diode Reverse Recovery Application Evaluation Results Efficiency of a SuperFET II FRFET MOSFET, FCH77N65F, a 65 V/77 mω is compared to 65 V / 8 mω competitor s fast recovery SJ MOSFET in 2 kw telecom AC/DC rectifier. As shown in Figure 14, Turnoff loss of FCH77N65F is 25% less compared to competitor MOSFETs at 1A drain current due to its low Q g. The summary of the efficiency measurements is shown Figure 15. Efficiency increases about.58% and.31% compared to competitor MOSFETs at light load and heavy load condition respectively. The major reason for higher efficiency of FCH77N65F is the reduced turnoff loss and output capacitive loss because of its lower Q g and E oss. Rev. 1. 7/3/15 5
Efficiency [%] Eoff [uj] AN5235 25 2 BestCompetitor FCH77N65F 15 1 5 5 1 15 2 25 3 35 Id [A] Figure 14. Turnoff Loss Comparison between SuperFET II FRFET MOSFET, FCH77N65F and 65 V, 8 mω competitor s fast recovery MOSFET 94 92 9 88 86 Best Competitor FCH77N65F 2 4 6 8 1 12 14 16 18 2 22 Pout [W] Figure 15. Efficiency versus output power in 2 kw telecom power supply between SuperFET II FRFET MOSFET, FCH77N65F and 65 V, 8 mω competitor s fast recovery MOSFET Conclusion The new 65 V fast recovery SuperFET II MOSFET combines a faster and more rugged body diode performance with fast switching performance, aimed at achieving better reliability and efficiency in power system applications including resonant converters. With reduced gate charge and stored energy in output capacitance, switching efficiency is increased and driving and output capacitive losses are decreased. Performance of fast recovery SuperFET II MOSFET allows designers to significantly increase system efficiency and reliability, particularly for in phase shifted fullbridge converters or halfbridge LLC resonant converters under abnormal conditions. Rev. 1. 7/3/15 6
AN5235 References [1] Wonsuk Choi and Dongkook Son New Generation SuperJunction MOSFETs, SuperFET II and SuperFET II Easy Drive MOSFETs for High Efficiency and Lower Switching Noise, Fairchild Application note, AN5232, Sept., 213. [2] Hangseok Choi, Analysis and Design of LLC Resonant Converter with Integrated Transformer, APEC 27, Feb. 27. [3] Wonsuk Choi and Sungmo Young, Improving System Reliability Using FRFET in LLC Resonant Converters, PESC 28, June 28. [4] Wonsuk Choi and Sungmo Young MOSFET Failure Modes Analysis in LLC Resonant converter, Fairchild Application note, AN967, Nov., 29 Author Wonsuk Cho, Dongkook Son and Dongwook Kim, Application Engineer HV PSS Team / Fairchild Semiconductor DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. Rev. 1. 7/3/15 7
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